Scrapie-like diseases occur in sheep, goats, cattle, and other ruminants, in mink, and in man. An identical disease has now been seen in cats. There are no in vitro laboratory tests of infection which thus can only me recognised clinically, neuropathologically, with tedious animal transmissions, and by finding an altered host protein, called PrP, electrophoretically or ultrastructurally. There is no immunity in these diseases, due to an absence of infection-specific antigens, Assays of infection depend on incubation period measurements in mice infected intracerebrally with serial dilutions of infective sources. Dilution of different strains or isolates, or of single strains in different host genotypes, or using different routes of infection, result in distinct dose-response curves (incubation period/infecting dose).